The present invention relates to a composite piezoelectric element used for an ultrasonic probe and the like, and a method of fabricating the composite piezoelectric element, and to an ultrasonic probe and an ultrasonic examination device employing the composite piezoelectric element.
A composite piezoelectric element is a piezoelectric material in which a piezoelectric element and a resin are compositely combined in various configurations. Such a composite piezoelectric element can widen a frequency band of oscillation. For this reason, it is studied that the piezoelectric element is utilized for an ultrasonic probe used in an ultrasonic diagnostic apparatus for medical purpose and the like. A composite piezoelectric element called as 1-3 type has a configuration in which a number of columnar piezoelectric elements are regularly or irregularly arranged, and gap portions between respective two of them are filled with a resin. It is pointed out that such a composite piezoelectric element of 1-3 type is suitable for sensitization and widening of band.
An ultrasonic probe used for diagnosing the interior of human body from the outside mainly uses an ultrasonic frequency band of about 3 to 10 MHz. In fabricating such a composite piezoelectric element of 1-3 type which transmits and receives ultrasonic waves in such a band, it is considered that the most excellent performances are attained when a ratio (L/S) is designed to be 5 or more, in the case where a length of a columnar piezoelectric element is L and a size of a section perpendicular to a longitudinal direction of the columnar piezoelectric element is S. Accordingly, when the composite piezoelectric element of 1-3 type is applied to the ultrasonic probe having a frequency band of about 3 to 10 MHz, it is necessary to form a configuration in which a number of columnar piezoelectric elements having a length L of about 160 to 500 μm and a section size S of about 30 to 100 μm or less are arranged.
In this specification, the ratio (L/S) is referred to as “an aspect ratio of a columnar piezoelectric element”. When the section of the columnar piezoelectric element is a circle, the size S is a diameter of the circle. When the section of the columnar piezoelectric element is a rectangle, the size S is a length of the longer side. In the case where the section of the columnar piezoelectric element is a trapezoid, a length of the lower side is referred to as the size S.
The availability of the composite piezoelectric element is known for a long time. However, there are not so many examples which are commercially available up to the present time. The main reasons are as follows: (1) the required configuration of a columnar piezoelectric element is extremely fine, so that it is difficult to fabricate a composite piezoelectric element, and (2) even though the fabrication is possible, a high fabrication cost is required.
In recent years, in order to diagnose the possibility of narrowing of blood vessel from the interior of the blood vessel, an ultrasonic diagnostic probe of a high frequency (15 to 20 MHz) with which observation in detail of a wall of blood vessel can be performed is required. When a composite piezoelectric element which can oscillate in such a frequency band is to be fabricated, it is necessary to form a configuration in which a number of columnar piezoelectric elements each having a length L of about 80 to 100 μm, a section size S of about 16 to 20 μm or less, and an aspect ratio of 5 or more are arranged. However, it is seriously difficult to fabricate a composite piezoelectric element having such a configuration by a conventional fabrication method.
Hereinafter, a conventional fabrication method of a composite piezoelectric element of 1-3 type will be described.
Japanese Patent Publication No.1789409 and Japanese Patent Publication No.1590342 disclose a method of manufacturing a composite piezoelectric element of 1-3 type. According to the method, after cutting grooves are formed longitudinally and latitudinally by machining a block-like piezoelectric element, the cutting grooves are filled with an organic high polymer such as an epoxy resin and then the organic high polymer is hardened, thereby forming the composite piezoelectric element. This method is referred to as “dice and fill”. The cutting grooves are formed by mechanical working such as dicing.
Japanese Patent Publication No.5-33836 discloses a fabricating method utilizing laser cutting instead of the dicing performed in the dice and fill method. In this method, after grooves are formed in piezoelectric ceramic with laser light, the grooves are filled with a resin and then the resin is hardened.
Both of the above-identified prior arts can be applied to the fabrication of a composite piezoelectric element used for an ultrasonic probe up to about 10 MHz, but can hardly be applied to the fabrication of a composite piezoelectric element used in a high frequency band equal to or higher than 10 MHz. Even in the case where the prior arts are applied to the ultrasonic probe up to about 10 MHz, the fabrication is extremely difficult, or even though the fabrication is possible, the fabrication cost is disadvantageously high.
Other fabrication methods of a composite piezoelectric element include a method disclosed in “IEEE 1997 ULTRASONIC SYMPOSIUM, pp. 877-881, 1997” (hereinafter referred to-as a prior-art document 1), and a method disclosed in “IEEE 1998 Microelectro Mechanics Systems Workshop, pp. 223-228, 1998” (hereinafter referred to as a prior-art document 2).
The fabrication method of the prior-art document 1 is as follows:
First, a resin mold having holes with high aspect ratio is formed by deep lithography using X rays. The holes are filled with ceramic slurry. Thereafter, the resin is removed by etching or the like, and then the ceramic is sintered. In this way, it is possible to produce a configuration in which a number of fine columnar piezoelectric elements with high aspect ratio are arranged. When gaps of the columnar piezoelectric elements in this configuration are filled with an organic high polymer, a composite piezoelectric element of 1-3 type can be produced.
The fabrication method of the prior-art document 2 is as follows.
Holes with high aspect ratio is formed in a silicon substrate by deep etching. The holes are filled with ceramic slurry. Thereafter, the ceramic is sintered while the holes in the silicon substrate are filled with the ceramic. After the sintering, the silicon substrate is removed by etching or the like. Thus, a configuration in which a number of fine columnar piezoelectric elements of high aspect ratio are arranged. Thereafter, gaps between the respective columnar piezoelectric elements are filled with an organic high polymer, thereby producing a composite piezoelectric element of 1-3 type.
The above-identified prior-art documents describe that it is possible to form a composite piezoelectric element having a section size equal to or smaller than 20 μm. However, both of the production methods include complicated processes, and the burning of mold requires a complicated process and a long time. In addition, a production apparatus to be utilized is expensive. As a result, the increase in production cost is a serious problem.
Moreover, lead zirconate titanate-based piezoelectric ceramics (PZT) with high piezoelectric performances is generally used as a piezoelectric element of a composite piezoelectric element. However, since the PZT is a ceramic including lead of low volatilizing temperature, the composition control is difficult, and it is difficult to perform the sintering so as to exhibit sufficient piezoelectric characteristics.